Does this do what you want?
#!/bin/bash
FILES=($(find /capture/nop/ -maxdepth 1 -name "*.pcap" -mtime +5))
# Discard path name and extension
TAG1=$(basename "${FILES[0]}" | sed 's/.pcap//')
TAG2=$(basename "${FILES[-1]}" | sed 's/.pcap//')
mergecap -w "output_from_${TAG1}_to_$TAG2" ${FILES[@]}